1. Field of the Invention
The present invention relates to an image formation-type soft X-ray microscopic apparatus with high resolving power which is mainly used for observing organisms.
2. Related Background Art
X-ray microscopes hitherto proposed are roughly divided into the following four types:
(1) A projection enlargement type which has no optical system and in which a sample is placed at a position near an X-ray point source in the divergent pencil of the X-rays generated from the X-ray source, and an X-ray film or a two-dimensional X-ray detector is disposed at a position behind the sample at a distance therefrom.
(2) An adhesion type which has no optical system and in which an X-ray source, which supplies a bundle of substantially parallel X-rays, is used, and a resist is caused to adhere as a sample to the X-ray source. In this case, a synchrotron radiation source (referred to as "SR" hereinafter), a plasma X-ray source or an electron beam excitation X-ray source is used as the X-ray source.
(3) A scanning type in which an X-ray beam is narrowed into a small spot by an optical system, and the beam and a sample are relatively scanned. In this case, SR is used as an X-ray source, and a Fresnel zone plate (referred to as "FZP" hereinafter), a multilayer film mirror or a total reflection mirror is used as an optical element for narrowing the X-ray beam into a small spot.
(4) An image formation type in which X-rays are condensed on a sample by using an X-ray source comprising SR, a plasma X-ray source or an electron beam excitation X-ray source and an optical element such as FZP, a multilayer film mirror or a total reflection mirror, and an image of the sample is formed on a film, a fluorescent plate or a two-dimensional X-ray detector by using the same optical element.
High-resolution observation of living organisms cannot be easily made by the above-described conventional X-ray microscopes from the technical viewpoint because the microscopes are insufficiently optimized and applies large amounts of X-rays.
Namely, although the projection enlargement type (1) is required to have a high-luminance X-ray point source, exposure for a long time is required because of its insufficient intensity, and dynamic observation is thus difficult. In addition, since the sample must be sliced in order to avoid a deterioration in resolving power caused by the influence of Fresnel diffraction, it is difficult to observe a living sample.
Since the adhesion type (2) has no high-resolution detector other than the resist, the development of the resist is necessary, and real time observation is thus difficult. In addition, since the magnification is 1, enlargement observation separately using an electron microscope or the like is required. Further, destructive observation, in which the sample is sliced, is required for avoiding a deterioration in resolving power caused by the influence of Fresnel diffraction in the same way as the projection enlargement type (1).
The scanning type (3) has the disadvantage that, since an X-ray source having good directivity is required, a large X-ray source such as SR must be used, and the size of the apparatus is significantly increased. In addition, since the scanning time, i.e., the exposure time, is long for obtaining a desired image, dynamic observation is difficult.
Since the image formation type (4) exhibits a low degree of efficiency when FZP is used, a large X-ray source such as SR must be used as a high-intensity X-ray source. In addition, the image formation type (4), which uses a mirror, has a disadvantage in that the resolving power cannot be easily improve, and the size of the optical system is increased. This type is thus insufficiently optimized.